The present invention relates to digital transmission, and more particularly to decoding a binary signal which is encoded by alternate mark inversion. Coded mark inversion (C.M.I) is a bi-phase two level transmission code known in full as: "Binary coded alternate mark inversion". This code is defined as follows:
______________________________________ Binary value C.M.I. value ______________________________________ 0 01 1 11 or 00 alternately ______________________________________
This provides a signal having no DC component, and which is transparent to a clock rate signal recoverable from the 1 to 0 transitions which always occur at the end of a bit. Further, since it is easy to implement both for coding and for decoding, it is well adapted for data transmission between equipments operating at a very high binary rate. It is described in particular in contribution No. 14 of the CCITT's special commission D in February 1974 together with a coder and a decoder for its implementation.
The decoding method used in the decoder described in the above article consists in spotting absences of a transition in the received signal by comparing the received signal in an "exclusive nor" logic gate with a version thereof that has been delayed by one-half of a bit period. This method provides a signal in which the binary data is only valid during the second half-period of each bit. This signal is transformed into non return to zero binary data (N.R.Z.-L) by means of a D-type bistable synchronized on the recovered clock rate. This has the drawback, as will be seen below, of requiring special synchronization conditions between the transitions in the received signals and those in the recovered clock rate signals. Meeting these synchronization conditions requires the phase position of the clock rate signal to be rigorously determined during regeneration and greatly limits the maximum tolerable amplitude the jitter in the received signal.
Preferred embodiments of the present invention mitigate the above-mentioned drawback and, consequently, to provide a considerable increase in the maximum amplitude of jitter which can be tolerated in the signal to be decoded.